The invention relates to an effective and environmentally safe process for the microbial leaching of sulfidic materials, particularly of sulfide ores such as pyrite, marcasite, chalcopyrite, bornite, or covelline, which process is characterized in that the aqueous leaching fluid is added with sulfur-containing amino acids or derivatives thereof. The invention is also directed to the use of sulfur-containing amino acids or derivatives thereof in the microbial leaching of sulfidic materials, particularly in pyrite leaching.
Microbial leaching is a well-known process in biohydrometallurgy for leaching out metals from ores and other mineral raw materials through the action of microorganisms. Obligatorily chemolithoautotrophic Thiobacillus species such as T. ferrooxidans and T. thiooxidans whose energy sources are sulfides, elemental sulfur and soluble thiosulfates, but also iron(II) ions as an alternative, play a central role in the chemistry of ore leaching. In any case, the microbial action leads up to the sulfate. As an example, reference is made to U.S. Pat. No. 2,829,964 wherein a cyclic leaching process using iron-oxidizing bacteria is described. In this process, the ore is leached with a sulfuric Fe(III) sulfate solution obtained using bacteria. Thereafter, leach and gangue are separated, the leach metal content is extracted, and the Fe(II)-containing final leach is re-oxidized using bacteria.
The literature also suggests a number of processes to improve ore leaching and, in particular, to increase the leaching rate, which processes, above all, envisage the use of surface-active substances (D. W. Duncan, P. C. Trussell, and C. C. Walden, Leaching of Chalcopyrite with Thiobacillus ferrooxidans: Effect of Surfactants and Shaking, 1964, Applied Microbiology 12(2), 122-126; I. Palencia, F. Carranza, and J. Pereda, Influence of Block Copolymers on the Microbiological Leaching of Pyrites by Discontinuous Operation, 1984, Tenside Detergents 21(2), 90-93; N. Wakao, M. Mishina, Y. Sakurai, and H. Shiota, Bacterial Pyrite Oxidation III. Adsorption of Thiobacillus ferrooxidans Cells on Solid Surfaces and Its Effects on Iron Release from Pyrite, 1984, J. Gen. Appl. Microbiol. 30, 63-67).
It is believed that these substancesxe2x80x94chiefly surfactants or polysaccharides and peptides or proteinsxe2x80x94improve the contact between bacteria and sulfide, thereby facilitating bacterial attack.
However, the surfactants that are used are disadvantageous because they have lacking or low biodegradability and do not represent environmentally safe substances. Peptides and proteins as surface-active compounds exhibit controversial effects in the leaching process.
It was therefore the object of the present invention to provide an effective leaching process with improved dissolution rate without the use of environmentally hazardous additives.
Surprisingly, it has now been found that bacterial attack by Thiobacillus species on sulfidic materials can be greatly accelerated by adding the aqueous leaching fluid with an amino acid, selected from cysteine, methionine, or derivatives thereof, or a mixture of these compounds at low concentration.
More specifically, homocysteine and amides or esters of cysteine, methionine or homocysteine are possible as derivatives which can be used in the leaching process according to the invention. Homocysteine is a derivative of methionine wherein the methyl group on the sulfur has been replaced by hydrogen, so that homocysteinexe2x80x94like cysteinexe2x80x94has a sulfhydryl group. According to the invention, both racemates and optically active forms of the amino acids may find use.
It has been found that an optimum effect is achieved in those cases where the concentration of the added amino acid(s) or derivatives thereof in the aqueous leaching fluid is low, not exceeding 8xc3x9710xe2x88x923 M, in particular. Concentrations of from 8xc3x9710xe2x88x924 to 8xc3x9710xe2x88x925 M are particularly preferred. The pH value of the leaching fluid is adjusted to 1.0-4.0, preferably to 1.5-2.0, and more preferably to 1.6. Adjustment is effected using suitable buffer solutions, e.g. Tuovinen buffer (Arch. Mikrobiol, 88, 285-298 (1973)).
According to the invention, there are two possible ways of performing the leaching process. On the one hand, the leaching fluid may include both the Thiobacillus species and the sulfur-containing amino acids or derivatives thereof. This embodiment is the preferred one. Alternatively, it is also possible to use the dilute amino acid solution alone as leaching fluid and subsequently add the thiobacilli to the discharging fluid (e.g. outside the dump) which is recycled. These two possible ways do not exclude the principal methods of ore leaching well-known to those skilled in the art, i.e., slope leaching, dump leaching or in situ leaching. In practice, the process according to the invention can be used with any of the three ore leaching processes.
According to the invention, T. ferrooxidans is preferred as Thiobacillus species. This strain is acidophilic, occurring in acidic waters of ore mines. Detailed investigations relating to the growth of this strain have also been described by Tuovinen O. H. et al. in Arch. Mikrobiol. 88, 285-298 (1973).
Thus, the process of the invention provides an effective method of ore leaching, particularly of pyrite, which process, in contrast to current methods such as cyanide leaching, does not represent any risk for the ecological balance of the environment. The amino acids and their derivatives used according to the invention are environmentally safe and inexpensive starting materials. They are employed at exceedingly low concentrations and result in an essential improvement of bacterial dissolution of metal sulfides (e.g. FeS2). Thus, for example, the process according to the invention permits speeding up the microbial recovery of copper or gold from pyrite ores.
The invention is also directed to the novel use of sulfur-containing amino acids, derivatives or mixtures thereof in the microbial leaching of sulfidic materials, particularly of sulfide ores.
With reference to the embodiments, the invention will be illustrated in more detail below.